Metallized electroceramic heating devices are generally made from doped barium titinate ceramics which have a sharp positive temperature coefficient of resistance. The ceramics are designed such that below a critical temperature, the resistance of the ceramic remains at a low value and is essentially constant. When a particular temperature is reached, a crystalline phase change takes place in the ceramic and this change in crystalline structure is accompanied by a sharp increase in the resistance at the crystalline grain boundaries. The result of this crystalline change is an increase in the heater resistance of several orders of magnitude over a small temperature change. For example, barium titinate heaters with a room temperature resistance of 3.0 ohms can increase to 1,000 ohms or more during the crystalline phase change. The temperature at which the cyrstalline phase change takes place can be adjusted in the manufacturing process through the use of appropriate chemical additives and can attain temperatures as high as 300.degree. C., and even higher.
Commonly, current is carried to the device by means of wires that are attached to the metallized coatings which form the electrodes on the electroceramic body. If attachment is by way of soldering, when making devices that can operate temperatures as high as 300.degree. C., special solders or solder pastes frequently have to be used, together with special fluxes. These special fluxes may seriously harm the devices since they can impregnate the grain boundaries of the ceramic when the device is operated and cause detrimental changes to the resistivity of the heater.
While the soldering method is the most common way to attach leads to the heaters, other methods are also used. Such methods include resistance welding and laser welding and each of these methods will produce a satisfactory product. Resistance methods, however, are somewhat limited because of the necessity of compatibility of melting points, coefficients of expansion and chemical compositions. Such requirements can pose severe limitations on the selection of materials necessary to build the heater. On the other hand, laser welding methods require the use of extremely expensive equipment to produce a relative inexpensive product.